The present invention relates to an upward tracing system for tracer control equipment which traces the surface of a model by means of a tracer head, and more particularly to an upward tracing system which clamps only a downward feed of the tracer head over a hollow in the model surface so as to prevent a cutter from going down.
In profiling through use of a face mill cutter or the like, the cutter will be broken when it descends. To avoid this, it is customary in this kind of profiling to employ an upward tracing system which executes a clamp feed when the cutter is about to go down during tracing, thereby inhibiting the cutter from descending.
Conventionally, the upward tracing system initiates the clamp feed upon detecting that the velocity component in the vertical axis (usually the Z-axis) of the tracking direction, which is obtained by calculation from signals available from the tracer head tracing the model surface, has come to assume a negative value indicating descent of the cutter. However, this system possesses such defects as follows:
(1) In general, a stylus of the tracer head performs a transient, up-and-down pulsating movement at the start of tracing. As a result, even if the model surface is horizontal or flat, the velocity component in the vertical axis may sometimes go negative at the start of tracing, in which case the clamp feed takes place, inducing a tracing error.
(2) In the case where the model surface gently slopes down, the distances of the downward movement of the tracer head is appreciable, but the negative value of the velocity component in the vertical axis is so small that it is difficult to detect with a high degree of accuracy. Especially, in the case of effecting the clamp feed when the velocity component in the vertical axis goes down below a certain negative value so as to avoid the disadvantage mentioned above in (1), it is difficult to inhibit the cutter from descending when the model surface gently slopes down.
(3) The tracing velocity obtained by calculation is applied via an amplifier circuit or the like to a servo motor. In this case, even if the velocity component in the vertical axis is zero due to aging of the amplifier circuit or the like, the cutter may sometimes go down. Therefore, the detection of a descent of the cutter according to the value of the velocity component in the vertical axis is inevitably affected by the aging of the amplifier circuit or the like, making reliable upward tracing difficult.